The present invention relates to a vortex flowmeter, and in particular to a vortex flowmeter that is capable of measuring flow speed and flow rate both rapidly and precisely.
While the vortex flowmeters available commercially are designed with strut-type vortex shedders, [refer to H. V. Mangin, Tappi, 58(5), 65(1975); T. J. S. Brain and R. W. W. Scott, J. Phys. E: Sci, Instrum., 15, 967(1982); and C. L. Smith, Chemical Engineering, 85, 155(1978)] researchers in this area are aware that a possible design of the vortex flowmeters is to consider the axisymmetric-type bluff bodies as vortex shedders. A pioneering work along this direction is due to Takamoto and Komiya (refer to Takamoto, M. and Komiya, K. "A Vortex Ring Shedding Flowmeter", Bulletin National Research Lab. Metrology, Vol.32, pp.125-134, 1983) who studied the phenomenon of vortex shedding of a ring situated in a free flow and in a circular pipe respectively. They found that for certain sizes of rings situated in the pipe, the quality of the vortex shedding signal obtained can be superior to that obtained behind the strut-type bluff body. They further suggested that a ring-type vortex flowmeter can be designed with a hot-wire of 10 .mu.m in diameter spanning between the inner edge of the ring to sense the periodic velocity fluctuations.
Cousins et al. [refer to T. Cousins, A. J. T. Hayward, and R. Scott, Flow Measurement FLOMEKO, the 5th International IMEKO-Conference on Flow Measurement FLOMEKO, VDI BERICHTE 768, 151 (1989)] proposed a design for a vortex flowmeter that has two rings in tandem arrangement. They compared four types of ring models, based on a number of criteria to evaluate the quality of the velocity signal measured. The vortex shedding frequency was reduced from the velocity signals obtained by a hot-wire probe situated in the flow field.
Miau and Liu [refer to Miau, J. J. and Liu, T. W. "Vortex Flowmeter Designed with Wall Pressure Measurement", Rev. Sci. Instrument 61(10), pp.2676-2681, 1990] proposed a disk-type bluff body as the vortex shedder. They found that the vortex shedding frequency was measureable in the wake of the disk for the geometrical blockage ratio of the disk up to 29.2%. Based on the characteristic velocity and length scales they suggested, the vortex shedding frequencies corresponding to different sizes of disks can be non-dimensionalized into a unified relation. In addition, they pointed out that the advantage of using the axisymmetric vortex shedder is that the vortex shedding frequency can be obtained by wall pressure measurement thus, avoiding the complications involved in the installation of a sensor into the vortex shedder or in the flow field.
The present invention concentrates on the ring-type bluff bodies as the vortex shedders and is somewhat different from the previous studies, that is, the studies suggested by Takamoto and Komiya and Cousins et al. [refer to T. Cousins, A. J. T. Hayward, and R. Scott, Flow Measurement FLOMEKO, the 5th International IMEKO-Conference on Flow Measurement FLOMEKO, VDI BERICHTE 768, 151 (1989)]. The sizes of rings interested in the present work fall in the regime that the process of periodic vortex shedding behind the ring is strongly influenced by the presence of the wall. The interaction between the wake flow of the ring and the presence of the wall is expected to induce wall pressure fluctuations significantly. Thus, similar to Miau and Liu [refer to Miau, J. J. and Liu, T. W. "Vortex Flowmeter Designed with Wall Pressure Measurement", Rev. Sci. Instrument 61(10), pp.2676-2681, 1990], vortex shedding frequency can be obtained with a pressure sensor situated on the wall of the pipe.